Corrosion resistance pipelines for the marine or otherwise underwater transportation or conveying of corrosive fluids such as gas or crude oil can be provided by pipes having an internal metallic liner. A double-walled or bi-metallic pipe is generally composed of two metallic layers. The outer layer is for resisting hydrostatic pressure, and/or internal pressure depending on the water depth, whilst the internal layer prevents damage to the outer layer from the chemical composition of the fluid being conveyed. The inner layer is sometimes also termed a “liner”. As one of its main purposes is to protect the outer layer from corrosion, commonly a corrosion resistant alloy (CRA) is chosen for the liner.
One form of bi-metallic pipe is a single “clad” pipe having an internal CRA layer metallurgically bonded to the outer layer, which could be formed from a carbon steel base metal.
A second form of bi-metallic pipe can be termed a mechanically lined pipe (MLP) formed from a number of pipe stalks in which a liner is fixed to the outer layer such as carbon steel without metallurgical bonding. An economic method of forming a lined pipe stalk in this way uses hydraulic expansion, where the liner is inserted into the outer layer, and then both parts are expanded. During the expansion, the inner layer undergoes a plastic deformation while the outer layer undergoes either an elastic or a plastic deformation, depending on the manufacturing process. One example of this comprises inserting an alloy 316L clad liner inside a carbon steel host pipe, and expanding the liner radially so that it comes into contact with the host pipe, and then the host pipe outer diameter will also expand together with the liner to a pre-determined strain level such that, following relaxation of the internal pressure, an interference contact stress between the liner and the host pipe remains.
Such bi-metallic pipe stalks are generally manufactured in 12 m or 24 m lengths at a manufacturing location, and then subsequently transported to a base or site next to a river or sea for welding together into conventionally ‘standard’ 1 km lengths.
However, corrosion can occur on the inner surface of the outer host pipe layers during the transportation and storage of such pipe stalks before use. Various manufacturers and suppliers of such pipe stalks have different ways of overcoming this, although the commonest method involves welding each end of the inner liner to the ends of the outer pipe to seal them. For example, the steel pipe shown in EP0150041A2 has a double construction in which an outer carbon steel pipe member is lined with respect to its whole length with a corrosion resisting material consisting of Ti.
The bi-metallic pipe stalks are conventionally transported and assembled on site at a spool base by the completion of circumferentially butt-welds between the pipe stalks. The material used to perform the welds is generally the same as the material used for the inner liners to ensure continuity of the liner material in the region of the welds. Thus, for example, when a corrosion resistant alloy is selected for the inner liner for an application in the sour service (i.e. in environments containing elemental sulfur), it is important that the property of the liner material is maintained all along the length of the liner. For this reason, all the welds of such 12 m or 24 m bi-metallic pipes are carried out with consumables formed from the corrosion resistant alloy material.
However, such consumables are higher in cost than other types of consumables, such as those formed only from carbon steel. Moreover, during the stalk-welding process, the materials used for the inner liner and the outer host pipe can mix and form an alloy. Thus, the integrity of the inner liner may not be maintained at all the weld locations despite the use of consumables of the same material of the inner liner. Any loss in integrity anywhere along the pipeline is of course catastrophic for the whole pipeline.
EP0150041A2 proposes a method of manufacturing corrosion resisting steel pipes of “limited” lengths after pipe-manufacturing and transportation to a building site, based on using end tubes so that a Ti metal overlaps the outer pipe members prior to welding.
GB2275639A describes welding its bi-metallic tubular members to form a pipe so that the butt welds formed do not contact the outer layers, followed by the addition of circumferentially lap welding sleeves around each weld not contacting the inner layers.
Such welding processes still take time and involve more complicated welding processes and additional materials such as sleeves than is desired in practice in order to avoid one or more of the above problems.